1. Field of the Invention
The present invention relates generally to control elements used in electronic circuits and, more specifically, the present invention relates to control elements with high voltage power transistors.
2. Background Information
Two of the primary goals in the design of control elements with integrated power transistors are cost and performance. Cost is generally reduced when the number of external components required in the electronic circuit are reduced, and when smaller, more efficient power transistors are employed. Performance may be improved by adopting a more efficient power transistor, which increases efficiency, and by lowering the manufacturing variance, which allows better control of critical parameters such as the peak current delivered by the power transistor.
A power supply is an example of an electronic circuit that utilizes a control element with power transistor. The start-up function of one known power supply is performed by a resistor, which provides high voltage DC from a bridge rectifier to the control circuit. Unfortunately, the start-up function resistor is expensive, requires a large area in the power supply and lowers supply efficiency by dissipating power continuously, even after the start-up function is completed. A current limit function of the known power supply is provided by a sense resistor that is in series with the source of the power transistor. Drawbacks of this known approach are the cost, size and power dissipation of the sense resistor that is in series with the source of the power transistor.
In another known power supply, a voltage regulator internal to the power supply chip is used to replace the start-up function resistor described above. The voltage regulator in the power supply chip may be turned off after the start-up function is completed, thus eliminating the extra power dissipation inherent to the power supply described above. However, the voltage regulator in the power supply chip consumes a significant area on power supply chip and is also prone to electrical static discharge (ESD) and safe operating area (SOA) damage.